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Physical Properties Physical properties of the alkenes and alkynes are quite similar to those of alkanes –Nonpolar –Not soluble in water –Highly soluble in nonpolar solvents Boiling points rise with molecular weight 11.1 Structure and Physical Properties

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11.2 IUPAC Names Base name from longest chain containing the multiple bond Change from -ane to -ene or -yne Number from the end, that will give the first carbon of the multiple bond the lower number Prefix the name with the number of the first multiple bond carbon Prefix branch/substituent names as for alkanes

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Molecules With More Than One Double Bond Alkenes having more than one double bond: –2 double bonds = alkadiene –3 triple bonds = alkatriene Same rules for alkynes 11.2 Nomenclature

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Naming Cycloalkenes 5-chloro-3-methylcyclohexene Name : 11.2 Nomenclature Cyclic alkenes are named like cyclic alkanes –Prefix name with cyclo Numbering must start at one end of the double bond and pass through the bond Substituents must have the lower possible numbers –Either number clockwise or counterclockwise

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Naming Haloalkenes Double or triple bonds take precedence over a halogen or alkyl group –2-Chloro-2-butene –If 2 or more halogens, indicate the position of each 11.2 Nomenclature

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11.3 Geometric Isomers: A Consequence of Unsaturation Carbon-carbon double bonds are rigid –Orbital shape restricts the rotation around the bond –Results in cis-trans isomers –Requires two different groups on each of the carbon atoms attached by the double bond

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Naming Geometric Isomers 2-butene is the first example of an alkene which can have two different structures based on restricted rotation about the double bond trans-2-butene cis-2-butene 11.3 Geometric Isomers: Consequence of Unsaturation

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Identifying cis/trans Isomers If one end of the C=C has two groups the same, cis-trans isomers are not possible Both carbons of the C=C must have two different groups attached Find a group common to both ends of the C=C –If the common group is on the same side of the pi bond, the molecule is cis –If on the opposite side, the molecule is trans 11.3 Geometric Isomers: Consequence of Unsaturation

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Questions to Identify cis-trans Isomers 1.Are both groups on a double-bond carbon the same? 1.A = B? C = D? If no, continue 2.Is one group on each carbon the same? A = C or D? B = C or D? If either or both is yes, cis-trans isomer is present i.A  B ii.C  D So continue i.A = C Isomer! 11.3 Geometric Isomers: Consequence of Unsaturation A D C B

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Distinguishing cis-trans Isomers Each carbon has 2 different substituents –One substituent on each carbon is the same (Cl) –The 2 chlorine atoms are attached on opposites of the plane of the double bond = trans trans-1,2-dichloro-1-butene 11.3 Geometric Isomers: Consequence of Unsaturation

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11.4 Alkenes in Nature Alkenes are abundant in nature –Ethene is a fruit ripener and promotes plant growth –Polyenes built from the isoprene skeleton are called isoprenoids –Isoprene is the basic 5 carbon unit shown here The next slide shows some isoprenoids

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Isoprenoids – Distinctive Aromas 11.4 Alkenes in Nature

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11.5 Reactions Involving Alkenes and Alkynes There are two kinds of reactions typical of alkenes: –Addition: two molecules combine to give one new molecule –Redox: oxidation and reduction The two classes are not always mutually exclusive

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Addition: General Reaction A small molecule, AB, reacts with the pi electrons of the double bond The pi bond breaks and its electrons are used to bond to the A and B pieces Some additions require a catalyst 11.5 Reactions Involving Alkenes and Alkynes

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Hydrogenation: Addition of H 2 Hydrogenation is the addition of a molecule of hydrogen (H 2 ) to a carbon-carbon double bond to produce an alkane The double bond is broken Two new C-H bonds result Platinum, palladium, or nickel is required as a catalyst Heat and/or pressure may also be required 11.5 Reactions Involving Alkenes and Alkynes

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Halogenation: Addition of X 2 Halogenation is the addition of a molecule of halogen (X 2 ) to a carbon-carbon double bond to produce an alkane The double bond is broken Two new C-X bonds result Reaction occurs quite readily and does NOT require a catalyst Chlorine and bromine are most often the halogen added 11.5 Reactions Involving Alkenes and Alkynes

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Bromination of an Alkene 11.5 Reactions Involving Alkenes and Alkynes Left beaker contains bromine, but no unsaturated hydrocarbon Right beaker contains bromine, but reaction with an unsaturated hydrocarbon results in a colorless solution

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Unsymmetrical Addition Two products are possible depending how the 2 groups (as H and OH) add to the ends of the pi bond The hydrogen will add to one carbon atom The other carbon atom will attach the other piece of the addition reagent –OH (Hydration) –Halogen (Hydrohalogenation) 11.5 Reactions Involving Alkenes and Alkynes

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Hydration A water molecule can be added to an alkene –The addition of a water molecule to an alkene is called hydration Presence of strong acid is required as a catalyst Product resulting is an alcohol 11.5 Reactions Involving Alkenes and Alkynes

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Markovnikov’s Observation Dimitri Markovnikov (Russian) observed many acid additions to C=C systems He noticed that the majority of the hydrogen went to a specific end of the double bond He formulated an explanation 11.5 Reactions Involving Alkenes and Alkynes

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Markovnikov’s Rule When an acid adds to a double bond –The H of the acid most often goes to the end of the double bond, which had more hydrogens attached initially H-OH H-Cl H-Br 11.5 Reactions Involving Alkenes and Alkynes

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Hydration of Alkynes Hydration of an alkyne is a more complex process –The initial product is not stable Enol produced – both an alkene and an alcohol Product is rapidly isomerized –Final product is either Aldehyde Ketone 11.5 Reactions Involving Alkenes and Alkynes

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Hydrohalogenation An alkene can be combined with a hydrogen halide such as HBr or HCl The reaction product is an alkyl halide Markovnikov’s Rule is followed in this reaction 11.5 Reactions Involving Alkenes and Alkynes

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Alkene Reactions Predict the major product in each of the following reactions Name the alkene reactant and the product using IUPAC nomenclature 11.5 Reactions Involving Alkenes and Alkynes

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Addition Polymers of Alkenes Polymers are macromolecules composed of repeating units called monomers –Polymers can be made up of thousands of monomers linked together Many commercially important materials are addition polymers made from alkenes and substituted alkenes –Addition polymers are named for the fact that they are made by the sequential addition of the repeating alkene monomer 11.5 Reactions Involving Alkenes and Alkynes

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Some Important Addition Polymers of Alkenes 11.5 Reactions Involving Alkenes and Alkynes

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11.6 Aromatic Hydrocarbons Benzene’s structure was first proposed 150 years ago –A cyclic structure for benzene, C 6 H 6 –Something special about benzene Although his structures showed double bonds, the molecule did not react as if it had any unsaturation –Originally named aromatic compounds for the pleasant smell of resins from tropical trees (early source) –Now aromatic hydrocarbons are characterized by a much higher degree of chemical stability than predicted by their chemical composition Most common group of aromatic compounds is based on the 6-member aromatic ring, benzene

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Benzene Structure The benzene ring consists of: –Six carbon atoms –Joined in a planar hexagonal arrangement –Each carbon is bonded to one hydrogen atom Two equivalent structures proposed by Kekulé are recognized today as resonance structures The real benzene molecule is a hybrid with each resonance structure contributing to the true structure 11.6 Aromatic Hydrocarbons

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Benzene Structure – Modern Modern concept of benzene structure is based on overlapping orbitals –Each carbon is bonded to two others by sharing a pair of electrons –These same carbon atoms also each share a pair of electrons with a hydrogen atom –Remaining 6 electrons are located in p orbitals that are perpendicular to the plane of the carbon ring These p orbitals overlap laterally Form a cloud of electrons above and below the ring 11.6 Aromatic Hydrocarbons

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Pi Cloud Formation in Benzene The current model of bonding in benzene 11.6 Aromatic Hydrocarbons

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IUPAC Names: Benzenes Most simple aromatic compounds are named as derivatives of benzene For monosubstituted benzenes, name the group and add “benzene” chlorobenzene ethylbenzene nitrobenzene 11.6 Aromatic Hydrocarbons

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IUPAC Names: Benzenes For disubstituted benzenes, name the groups in alphabetical order –The first named group is at position 1 –If a “special group” is present, it must be number 1 on the ring An older system of naming indicates groups using –ortho (o) = 1,2 on the ring –meta (m) = 1,3 on the ring –para (p) = 1,4 on the ring 11.6 Aromatic Hydrocarbons

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Historical Nomenclature Some members of the benzene family have unique names acquired before the IUPAC system was adopted that are still frequently used today 11.6 Aromatic Hydrocarbons

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Benzene As a Substituent When the benzene ring is a substituent on a chain (C 6 H 5 ), it is called a phenyl group –Note the difference between Phenyl Phenol (a functional group) 4-phenyl-1-pentene 11.6 Aromatic Hydrocarbons

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Polynuclear Aromatic Hydrocarbons Polynuclear aromatic hydrocarbons (PAH) are composed of two or more aromatic rings joined together –Many have been shown to cause cancer 11.6 Aromatic Hydrocarbons

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Benzene Halogenation Halogenation places a Br or Cl on the ring –The reagent used is typically Br 2 or Cl 2 –Fe or FeCl 3 are used as catalysts 11.6 Aromatic Hydrocarbons

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Benzene Nitration Nitration places the nitro group on the ring Sulfuric acid is needed as a catalyst 11.6 Aromatic Hydrocarbons

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Benzene Sulfonation Sulfonation places an SO 3 H group on the ring –Concentrated sulfuric acid is required as a catalyst –This is also a substitution reaction 11.6 Aromatic Hydrocarbons

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11.7 Heterocyclic Aromatic Compounds Rings with at least one atom other than carbon as part of the structure of the aromatic ring –This hetero atom is typically O, N, S –The ring also has delocalized electrons The total number of atoms in the ring is typically either: –A six membered ring –Some have a five membered ring

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Heterocyclic Aromatics Heterocyclic aromatics are similar to benzene in stability and chemical behavior Many are significant biologically Found in DNA and RNA Found in hemoglobin and chlorophyll 11.7 Heterocyclic Aromatic Compounds